1. Field of the Invention
This invention pertains to the field of reinforced concrete construction and more specifically concerns improved splice joints between male and female threaded ends of steel reinforcement bars used in such construction.
2. Background of the Invention
In reinforced concrete construction it is commonly necessary to splice together steel reinforcement bars to meet the need for bar lengths longer than available stock, or to tie together adjacent sections of poured concrete. Such rebar splices are frequently made by providing mating threads at the rebar ends to be joined. One rebar end has a male screw threading, and a second rebar end is enlarged to form a socket with a female thread. A threaded splice can also be made by joining two male threaded rebar ends by means of an internally threaded sleeve. Threaded joints of this type include those with parallel threads, i.e. where the thread diameter is constant along its length, and tapered threads which diminish in diameter towards the end of the rebar in the case of male threading and where the female thread has a maximum diameter at the opening of the socket and diminishes in diameter towards its interior.
Tapered threads quickly separate as soon as they are loosened and the joint depends on tight frictional engagement between the male and female thread surfaces to preserve integrity of the rebar splice. Consequently, tapered threads require that the splice joint be torqued together in order to maintain the joint.
Parallel threads are not subject to this limitation, however, and the male/female threads remain in mutual engagement without being torqued together. Separation of the male/female threads requires that the male thread be actually fully unscrewed from the female thread, which does not occur simply as a result of loose engagement between the thread surfaces. This is reflected in the building codes presently in effect which do not require torquing of parallel thread rebar joints, and present industry practice in fact does not call for such torquing. Typically, the male thread is simply turned until the end of the rebar reaches the bottom of the female bore, a condition which under present practice is deemed to constitute a sufficient and adequate splice joint.
Rebar threads are cut or rolled into the steel bars to relatively low tolerances, due to the nature of the steel alloys used for manufacture of concrete reinforcement bars and the limitations of efficient high volume production of rebars at competitive cost. The result is that typical rebar thread surfaces have a significant degree of small-scale irregularity which at a micro level prevents full surface-to-surface contact between opposing male/female thread surfaces. These small scale irregularities do not normally reduce the tensile strength of the rebar splice joint, and current practice produces threaded joints which readily meet, for instance, building code requirements of a 60,000 psi yield strength and 90,000 psi ultimate tensile strength.
An aspect of threaded splice joints which has been largely overlooked until recently is the behavior of the splice joint under seismic or fatigue conditions where the joint is subjected to rapidly alternating tension and compression force cycles, i.e. where the load on the joint is rapidly and repeatedly reversed. Under such conditions, the spliced rebars do not perform as a single unbroken bar. For example, irregular contact between opposing thread surfaces in a parallel thread splice joint may be evidenced by clicking sounds as the joint is alternately subjected to stress and strain. If such a splice joint is embedded in a concrete structure, one or both of the rebars may slip axially relative to the surrounding mass of concrete when subjected to cyclic loading along the splice axis. Any movement of a rebar relative to the concrete it is intended to reinforce is undesirable and potentially weakens the structure.
Current building codes only specify that a rebar splice must exceed 125% of its yield strength under continuous load conditions without breaking. Rebar splices are not tested for slippage of the rebars relative to the concrete nor for splice joint performance under peak loads typical of seismic conditions.
What is needed is a threaded rebar splice joint which is more resistant to cyclic loading conditions, with performance more closely approximating that of an unbroken, continuous steel bar.